Masticatory muscle function and transverse dentofacial growth.

Numerous experimental and clinical studies have shown an association between masticatory muscle function and craniofacial growth. The present series of studies focuses on the influence of masticatory muscle function on some basic mechanisms in craniofacial growth as well as on the effect of the functional capacity of the masseter muscle on the width of the maxillary dental arch in human subjects. The animal experimental model used to study the influence of masticatory muscle function was a non-invasive one that produced a decreased functional demand of the masticatory system in growing rats by feeding them a soft diet. The influence of reduced masticatory muscle function on the transversal dimensions of the premaxilla, maxilla (including the dental arch) and the calvaria was studied on dry skulls using a high precision non-contact measuring system. Moreover, a methodological study was carried out to study the influence of one preparation method on the dimensions of dry rat skulls, using axial contact radiographs. The effect of different functional demands on sutural dimensions and sutural bone apposition was studied on frontal undecalcified sections of the rat snout using automatic image analysis. For the quantification of sutural dimensions contact microradiographs were used, while for sutural bone apposition the bone level at certain time intervals was marked with calcein. In the clinical part of this thesis the relationship between maxillary dental arch width and masseter muscle thickness in humans was studied using ultrasonography. Masticatory muscle function was found to influence the transverse growth of the skull at areas under direct muscle influence as well as the dental arch width in regions with molars under eruption. The methodological study indicated that direct comparisons between measurements on fresh specimens and those on dry rat skulls are not permissible, since the dimensions of the dry rat skull are smaller than those of the original fresh specimens. The dimensions and morphology of the facial sutures as well as the sutural bone apposition were negatively affected by reduced masticatory function. The experimental findings of the present study might indicate some of the underlying mechanisms of the clinical findings, where subjects with thicker masseter muscles were found to have a broader maxillary dental arch.